The present invention relates to the deposition of metal films using a low-pressure vacuum-based system, and more particularly to the deposition of metal films via chemical vapor deposition (CVD) processes in which accurate and reproducible control of the substrate temperature is necessary to ensure optimal properties of the metal films being deposited. Moreover, the present invention relates to a method and apparatus whereby substrates which may be subject to deformation upon heating can be uniformly and reproducibly heated to the desired processing temperature despite said deformation in a low-pressure cold-walled reactor in which metal films are deposited.
During the manufacturing of integrated semiconductor devices, it is oftentimes necessary to or desirable to deposit a metal film upon a substrate. For example, metal depositions may be employed for the formation of field effect transistor (FET) gate electrodes, metal oxide semiconductor (MOS) capacitors, read/write lines for dynamic random access memory (DRAM) devices, various contacts, multilevel wiring arrays and other various applications. An important method for effecting such deposition is CVD, especially in situations in which conformal coverage of a structured substrate is necessary.
In most typical CVD processes, the following four steps are essential: (1) The substrate to be coated must be placed within an enclosed vessel (i.e., reactor). (2) The substrate must be brought to the desired temperature to effect the intended deposition. (3) The substrate at temperature, T, will be exposed, typically under continuous flow conditions, to a gas mixture comprising precursor gas molecules (i.e., molecules containing metal atoms), any required co-reactant molecules such as reducing agents or other such molecules which might chemically react with the precursor molecules during deposition, and any carrier gas required to control the transport of reactive chemical species to and through the reactor. The total pressure of this entire gas mixture, which is referred to as P (i.e., the reaction pressure) herein, may or may not be constant during the deposition process. It is further understood that the composition of the gas mixture may also vary during the deposition process, i.e., the gas mixture may have differing proportions of carrier gas, precursor, and co-reactants. (4) The resultant substrate containing deposited metal film must be removed from the reactor.
It is very difficult, and in some instances, nearly impossible to achieve a uniform and reproducible substrate temperature T. This is especially the case under circumstances in which the requirements for producing metal films having requisite properties make it necessary to conduct the CVD process at a value of P of less than about 1 torr, and wherein the reactor employed is a cold-walled reactor. The term xe2x80x9ccold-walled reactorxe2x80x9d as used herein denotes any reactor which possesses surfaces within the line of sight of the substrate undergoing the deposition that are 50xc2x0 C. or more colder than the reaction temperature, Trxn.
In a prior art cold-walled reactor, the substrate is brought to the reaction temperature, Trxn, by means of a heater, which is typically a ceramic pedestal enclosure possessing internal heating elements situated within the reactor vacuum upon which the substrate is disposed. In such a system, the reaction temperature of the substrate is achieved by a dynamic balance of heat input from the heater into the substrate and heat loss from the substrate to the reactor surroundings, by, for example, radiative heat loss. Where the temperature difference between the reaction temperature and the reactor walls is large, the radiative heat losses of the substrate to its surroundings may be quite significant necessitating the operation of the heater at temperatures well above T, in order to bring the substrate disposed upon it to T. In such a dynamic situation, it is extremely difficult to reliably and reproducibly heat substrates uniformly to the desired substrate temperature.
The standard prior art method for providing a good reproducible thermal link between the heater and the substrate is by chucking. The term xe2x80x9cchuckingxe2x80x9d as used herein denotes any process which applies a force between the substrate and the heater (other than the force provided by weight of the substrate). Unfortunately, none of the prior art chucking schemes including mechanical, vacuum and electrostatic is applicable to low-pressure metal CVD processes.
Mechanical chucking, such as a clamping system, is always undesirable, as it produces a substrate with uncoated bare spots where the clamp touches the substrate. This is frequently intolerable in itself. Moreover, deposition on the clamping mechanism can give rise to particulate contamination.
Vacuum chucking is not viable because such chucking does not work at the low-pressures specified above and it is impossible to establish a sufficient absolute pressure differential to achieve adequate vacuum chucking.
Electrostatic chucking, which is suitable for deposition of insulating materials, is unsuitable for metal deposition because the deposition of conductive films, however minute, on the heater surface prevents the maintenance of the requisite static charge. Since the above chucking methods for controlling the thermal link between heater and the substrate are not applicable for low-pressure metal deposition processes, a procedure whereby one simply places the substrate upon the heater is typically employed.
With a simple substrate, such as an extremely flat wafer of bare silicon, simply placing the substrate upon the heater surface might suffice provided that the heater surface is extremely flat such that it contacts the entire substrate in the same way and that there is no significant temperature variations across the heater surface. However, a majority of substrates are more complex. For instance, typical substrates frequently comprise multilayered structures in which the individual layers comprise materials having different coefficients of thermal expansion. Even if such substrates were perfectly flat at room temperature, upon heating to reaction temperature (typically about 200xc2x0 to about 500xc2x0 C.) the substrates would substantially deform. This is the same physical principal responsible for the operation of ordinary bimetallic strip household thermostats.
FIG. 1 is a pictorial representation of a prior art method of heating a substrate by placing the substrate on the surface of the heater without any holding means. Specifically, FIG. 1 comprises reactor chamber 10 having heater 12 therein. As shown, substrate 14 is placed on the surface of heater 12 and during heating only the center portion of the substrate (labeled as 16 in the drawing) is heated by direct contact with the heater body. That is, the substrate tends to warp substantially during the deposition of metal films thereon. This phenomenon is caused by stresses resulting from having layers of various materials on the substrate that have different coefficients of thermal expansion. The only direct source of heat to the substrate edges (labeled as 18 in the drawing) is by radiation from the heater. Note the presence of gap 20 between substrate edges 18 and heater 10. When a multilayered substrate is heated to 400-500xc2x0 C. distortions up to a millimeter or so between the center and edges can be observed.
There are several problems associated with the situation depicted in FIG. 1. First, it is clear that the non-uniform power input into the substrate will lead to temperature variations across the substrate. These temperature variations, in turn, can produce undesirable variations in either thickness of the deposited metal films, its intensive properties or both. Second, even the average temperature on the substrate will be too sensitive to the details of the individual substrates. Different degrees of deformation will clearly result in different average substrate temperatures as the thermal link between the substrate and heater is being modified thereby. Since heating at the substrate edges may be largely radiative, the final substrate temperature may also depend upon the reflectivity of the back surface (e.g., polished vs. unpolished).
In short, the substrate temperature actually obtained could be a sensitive function of the composition and back surface finish of the substrate, as well as heater temperature. At the very least, this would necessitate frequent time-consuming recalibrations of the reactor every time substrates with even slightly different properties are introduced for deposition. In addition, the opening of a gap between the substrate edges and the heater will allow deposition of metal directly onto the surface of the heater, further changing its thermal transfer characteristics as well as being a potential source for particulate contamination.
In view of the drawbacks mentioned hereinabove with prior art metal chemical vapor deposition processes, there is a continued need for providing a new and improved CVD process in which accurate and reproducible control of the substrate temperature is provided thereby ensuring optimal properties of the deposited metal films. A new and improved CVD method is especially needed in cases wherein substrates that are subjected to deformation upon heating are employed.
One object of the present invention is to provide a low-pressure metal CVD process (on the order of about 1 torr or less) and apparatus in which accurate and reproducible control of the substrate temperature is provided.
Another object of the present invention is to provide a low-pressure metal CVD process and apparatus that work in cases wherein substrates that are typically susceptible to deformation (i.e., warping) upon heating are employed.
A further object of the present invention is to provide a low-pressure metal CVD process and apparatus that work when cold-walled reactors are employed.
A yet further object of the present invention is to provide a low-pressure metal CVD process and apparatus in which a reproducible thermal link between substrate and heater is maintained without the need of any mechanical chucking means, vacuum chucking means or electrostatic chucking means.
These and other objects and advantages are achieved in the present invention by introducing a chamfered ring disposed above the heater surface and by placing the substrate upon the chamfered ring rather than on the heater surface so that the substrate is resting on the internal chamfered surface. The term xe2x80x9cchamfered ringxe2x80x9d as used herein denotes a ring in which a groove or bevel is formed therein. Thus, in the present invention, the substrate is disposed above the surface of the heater (typically by a few millimeters) so that the substrate is now entirely heated by radiation by the heater instead of contact heating.
The chamfered ring, the top of the heater and the bottom surface of the substrate define a cavity that contains most of the radiation emanating from the heater. The integrity of this cavity is substantially preserved even if the substrate deforms in the manner described above. Thus, the thermal transfer between the heater and the substrate will, in contrast to the prior art, not be strongly affected by the deformation of the substrate. This leads to superior reproducibility and constancy of substrate temperature and also serves to obviate the problem of deposition on the heater itself.
In one aspect of the present invention, a method of depositing a metal film on a surface of a substrate using a low-pressure CVD deposition process is provided. Specifically, the method of the present invention comprises the steps of:
(a) positioning a substrate on a beveled surface of a chamfered ring that is located above a heater, wherein said heater and said chamfered ring are inside a chemical vapor deposition reactor chamber; and
(b) depositing a metal film on a surface of said substrate at pressures of about 1 torr or less, wherein during said depositing said substrate is not in physical contact with the heater except via the chamfered ring.
Another aspect of the present invention relates to a chemical vapor deposition (CVD) apparatus which comprises:
a reaction chamber including at least a heater and a chamfered ring having a beveled surface, wherein said beveled surface of said chamfered ring is located above said heater at a distance sufficient to form a cavity between said heater and a bottom surface of a substrate placed on said beveled surface so that the substrate is not in physical contact with the heater except via the chamfered ring.